SOLUBILITY OF COPPER IN A SULFUR-FREE MAFIC MELT

Abstract

The solubility of Cu in S-free mafic melts has been measured at a series of fO2 values and temperatures of 1245 and 1300°C. At constant temperature Cu solubility increases from 0.04 wt% at log fO2 = ?11.9 to 1.10 wt% at log fO2 = ?7.4. Copper solubilities were in excess of 8 wt% in two runs controlled at very high fO2 conditions of 10?1.4 and 10?2.2. Partitioning of Cu between metal and glass shows a strong fO2 dependence, with Dmet/glCu ranging from 90 at log fO2 = ?7.4 to 2190 at log fO2 = ?11.9. Slopes of Cu solubility and Dmet/glCu vs. log fO2 suggest that Cu dissolves predominantly in the +1 valence state. Copper solubility decreases with increasing temperature at constant fO2, similar to experimental results for Ni, Co, and Mo (Dingwell et al., 1994; Holzheid et al., 1994). The data are consistent with Cu dissolution as an oxide (represented by CuO0.5) and suggest that changes in fO2 (Fe2+/Fe3+ variations and Cu1+ complexation with Fe3+) may have large effects on the distribution of Cu between silicate and sulfide magmas. Results also suggest that the extraction of oxide-bonded Cu in mafic magmas by externally derived S may be an important mechanism in the generation of Cu-rich sulfide ores.